Yesterday on NEET 2019 exam a question was asked about rainbows... Without your superb lecture on bows, I don't think I could have done it. Thank you so much sir. I got 42/45 physics questions correct.
Sir, I had used a powerful tool 'coefficient of restitution (e) ' [e=1 ,for elastic collision] for my solution ...Though i know it basically comes from the conservation of kinetic energy and momentum ...It really makes life simpler 😀😀 :)
This is a very good problem with both solutions in that it really shows, how important it is to understand what happens and not just remember formulas and how to use them. I could have solved this in 5 seconds in my head, but being a bit rusty i wanted to verify that i got the concepts right before writing snd sending my answer. Often selecting the frame of recerence well makes things quite easy. Of course calculating this accurately would be very complicsted and it would require information of both balls materials, layer thicknesses, air pressure and such, but seeing that maximum is 9h is very simple, if you just understand what happens. This is the kind of physics everyone should have in their toolbox.
Dear professor, how is this ball drop different from a head on collision between the same basket ball and the tennis ball on the floor? Roll both of them at the same speed to hit each other head on. The result in my trial is that the tennis ball will speed off in the same speed as that of the basket ball, in the latter's direction. It is the same as that of the second collision in the ball drop - basket ball bouncing back and hitting the falling tennis ball. But why the resulting speeds are so different in both cases? One apparent factor is the friction, but is the difference in final speed only due to friction?
Mr. Walter Lewin, I like to research about this reality and I know that it is hard to accomplish my goal in my life time. So, I'm trying to learn Physics and Mathematics from my childhood. I love math & physics. But i don't know where to start. Please tell me what to learn first in order to understand calculus. So that i can see this whole universe in a mathematical framework.
Hello, I have a question on another topic. When calculating angular momentum (3D), and then the torque, which omega do you use in the cross product with the angular momentum? I have troubles finding a lecture about it. Thank you for your answer and your work is very inspirational.
I understand the solution but have a problem with the assumptions. Why/how is it that the tennis ball is still moving downward when the basketball has already reversed direction? In other words, how is it that the basketball has finished its bounce before the tennis ball has begun its own? I know these are approximations but it still must be MOSTLY true for the experiment to work.
I suggest you repeat the demo but record it with milli sec timing. That will tell the whole story. I do this demo in my 8.01 lectures have you watched that lecture? THe tennis ball almost hits the high ceiling in the lecture hall.
@@lecturesbywalterlewin.they9259 I was thinking how a high definition slow motion camera would make for a great video, but i dont have that technology, and I couldnt find anything on youtube that used that. I think the following logic is okay. As the basketball is being smushed into the ground, it accelerates upward. If the tennis ball and top of basketball could NOT be smushed at all, the tennis ball would have the same acceleration. But the tennis ball gets a little extra pillow so that it doesn't accelerate as drastically. Therefore the basketball hits velocity zero faster, and also achieves positive velocity before the tennis ball hits velocity zero. It is just confounding to me how that pillow allows the tennis ball to keep SO MUCH of its downward velocity, even until the time the basketball has SO MUCH of its upward velocity.
@@lecturesbywalterlewin.they9259 and yes I have seen that lecture and the demo is fantastic. Perhaps a youtube channel that specializes in slow motion videos would consider doing this demo today. Maybe I can convince them haha I will try
It seems to me the easiest solution would be this --- since the deformation energy of a basketball and tennis balls is expended on the kinetic energy of both balls in the same direction, and their masses differ 10 times, then the speeds will differ in the square root of 10, that is 3.162 times.
Sir, should we consider comparing the radii of the two balls as I have a question that how the tennis ball can fall down a height H. If the radius of basketball, upon which the tennis ball is kept, is considered, then the situation will not be that the tennis ball has fallen through a height H. Can you clear is doubt sir?
Make a drawing place a basketball with its lowest part h m above the ground and put a tennis ball on top. When the basketball hits the ground both the tennis ball and the basketball have fallen over a distance h.
I allege you are missing the fact that there are 3 masses and 2 collisions (you are not missing anything, but you are missing it in the explanation which could be misleading to some students). Let M0 be the mass of the earth. M0 >> M1, and M1>> M2. In both solutions, you quickly "brush off" any calculation of the first collision (the basketball on the ground), simply stating that is reverses direction and preserves speed. If the first collision's results are "so obvious", then the second should be also. If you are going to perform the "poor man's solution", that method needs to be used for BOTH collisions, or NONE of them, but not for one and not the other. So "poor man's solution" should start with M0 and M1, getting equations, then applying M0 >> M1 to CALCULATE that the basketball's velocity is unchanged in magnitude and reversed in direction, relative to earth. Then you apply the same equations again (except for M1/M2 instead of M0/M1) and apply M1 >> M2, and come to the same conclusion, relative to the basketball, and combining these to get V(M2) = - 3V(M1). If the mass ratios were not infinite, the math would get very messy, as 2 nasty formulas would need to be cascaded!
@@RM-zz2er many comments require no response from me. If you listen to the problem and also to my solutions you will probably agree that there is no need for me to respond.
Electric charges and fields chapter there in my syllabus can you tell me here through which lecture should i go on your channel for electric charges and fields
Good morning sir, I have a question, conservation of momentum is valid when the sum of all forces in the system is zero, but here it is always equal to -(m1+m2)g so what approximation are you using here to say that the net force is negligible?
If you compare the split moment before coll with the split moment after the coll there is no time for gravity to change the momentum and no time to change the KE. That's the meaning of conservation!!! watch my 8.01 lectures.
NO ONE CAN BEAT A PROFESOR WHO TEACHES ONE SUBJECT AT COLLEGE AS WELL AS EVEN AT SCHOOL LEVEL IF HE HAS MORE THAN 15 YEAR EXPERIENCE OF TEACHING !!!!!!!!!! WHY NOT YOU COME INDIA !!!!!!!! AS A GUEST LECTURE FOR STUDENTS !!!!!!!!
I was 4 times in India - My last 6 week visit was in 2014. I gave 8 lectures at 8 different Universities. March 5, 2014, a Very Special Day at IIT Kanpur in India. voxiitk.com/stalwarts-of-physics-at-techkriti/
@@lecturesbywalterlewin.they9259 But Sir Do you Know Physics Required For Me To Become an Quantum Physicist.It would be Nice of you SIr If you Could Help Me
@@alipoetry1626 *first get a bachelor's degree in Physics then a PhD in Quantum Physics at the best University that will accept you and take it from there*
Yesterday on NEET 2019 exam a question was asked about rainbows...
Without your superb lecture on bows, I don't think I could have done it.
Thank you so much sir.
I got 42/45 physics questions correct.
super, congratulations!
Sir, I had used a powerful tool 'coefficient of restitution (e) ' [e=1 ,for elastic collision] for my solution ...Though i know it basically comes from the conservation of kinetic energy and momentum ...It really makes life simpler 😀😀 :)
This is a very good problem with both solutions in that it really shows, how important it is to understand what happens and not just remember formulas and how to use them. I could have solved this in 5 seconds in my head, but being a bit rusty i wanted to verify that i got the concepts right before writing snd sending my answer. Often selecting the frame of recerence well makes things quite easy. Of course calculating this accurately would be very complicsted and it would require information of both balls materials, layer thicknesses, air pressure and such, but seeing that maximum is 9h is very simple, if you just understand what happens. This is the kind of physics everyone should have in their toolbox.
Your videos helped me understand physics in college and now my son in high school. Always fun to go back and revisit forgotten topics. Thank you!!!
great!
I'm really happy that my solution posted on the same day as prob. 61 matches with your master solution exactly.
Thank you very much professor. You are an excellent teacher and an inspiration to all of us who study physics on our own. Great job!
Thank you for sharing,great topic,great video.
Creative mind of 21st century
Professor black colour suits on you👍
Very informative and very inspiring...thank you for sharing your great work
Dear professor, how is this ball drop different from a head on collision between the same basket ball and the tennis ball on the floor? Roll both of them at the same speed to hit each other head on. The result in my trial is that the tennis ball will speed off in the same speed as that of the basket ball, in the latter's direction.
It is the same as that of the second collision in the ball drop - basket ball bouncing back and hitting the falling tennis ball. But why the resulting speeds are so different in both cases?
One apparent factor is the friction, but is the difference in final speed only due to friction?
Thanks for the solutions Sir. That was a very beautiful problem. Take care and looking forward for your next problem.
I like your solution better.
Yes I have done the same as first solution.:)
Oh professor what a solution the first 1 is. Pls start a series on S.S. Krotov. Plllls
is there anyone watching it in quarantine and thinking that he is an absolute genius
Mr. Walter Lewin, I like to research about this reality and I know that it is hard to accomplish my goal in my life time. So, I'm trying to learn Physics and Mathematics from my childhood. I love math & physics. But i don't know where to start. Please tell me what to learn first in order to understand calculus. So that i can see this whole universe in a mathematical framework.
Sir can you please solve the paradox given in Feynman lecture notes vol2 article 17.4.
I do not solve problems for viewers, I teach Physics.
Lectures by Walter Lewin. They will make you ♥ Physics.
Sir I just requested to explain the physics related to that particular article.
Hello, I have a question on another topic. When calculating angular momentum (3D), and then the torque, which omega do you use in the cross product with the angular momentum? I have troubles finding a lecture about it. Thank you for your answer and your work is very inspirational.
I cover this in my 8.01 lectures - watch them! Read the topics of the lectures to decide which will be best for you
Have a nice day, too
Thank you Sir
I understand the solution but have a problem with the assumptions. Why/how is it that the tennis ball is still moving downward when the basketball has already reversed direction? In other words, how is it that the basketball has finished its bounce before the tennis ball has begun its own? I know these are approximations but it still must be MOSTLY true for the experiment to work.
I suggest you repeat the demo but record it with milli sec timing. That will tell the whole story. I do this demo in my 8.01 lectures have you watched that lecture? THe tennis ball almost hits the high ceiling in the lecture hall.
@@lecturesbywalterlewin.they9259 I was thinking how a high definition slow motion camera would make for a great video, but i dont have that technology, and I couldnt find anything on youtube that used that. I think the following logic is okay. As the basketball is being smushed into the ground, it accelerates upward. If the tennis ball and top of basketball could NOT be smushed at all, the tennis ball would have the same acceleration. But the tennis ball gets a little extra pillow so that it doesn't accelerate as drastically. Therefore the basketball hits velocity zero faster, and also achieves positive velocity before the tennis ball hits velocity zero. It is just confounding to me how that pillow allows the tennis ball to keep SO MUCH of its downward velocity, even until the time the basketball has SO MUCH of its upward velocity.
@@lecturesbywalterlewin.they9259 and yes I have seen that lecture and the demo is fantastic. Perhaps a youtube channel that specializes in slow motion videos would consider doing this demo today. Maybe I can convince them haha I will try
It seems to me the easiest solution would be this --- since the deformation energy of a basketball and tennis balls is expended on the kinetic energy of both balls in the same direction, and their masses differ 10 times, then the speeds will differ in the square root of 10, that is 3.162 times.
your "easiest" solution is not correct.
Thank you!)
😍😍
Sir, should we consider comparing the radii of the two balls as I have a question that how the tennis ball can fall down a height H. If the radius of basketball, upon which the tennis ball is kept, is considered, then the situation will not be that the tennis ball has fallen through a height H.
Can you clear is doubt sir?
make a drawing - large R and small radius on top. When the large R hits the ground it has fallen over the EXACT same distance as the small r on top.
@@lecturesbywalterlewin.they9259 Sir, but it doesn't happen that the little r hits the ground exactly at a height h.....
Make a drawing place a basketball with its lowest part h m above the ground and put a tennis ball on top. When the basketball hits the ground both the tennis ball and the basketball have fallen over a distance h.
I allege you are missing the fact that there are 3 masses and 2 collisions (you are not missing anything, but you are missing it in the explanation which could be misleading to some students). Let M0 be the mass of the earth. M0 >> M1, and M1>> M2. In both solutions, you quickly "brush off" any calculation of the first collision (the basketball on the ground), simply stating that is reverses direction and preserves speed. If the first collision's results are "so obvious", then the second should be also. If you are going to perform the "poor man's solution", that method needs to be used for BOTH collisions, or NONE of them, but not for one and not the other. So "poor man's solution" should start with M0 and M1, getting equations, then applying M0 >> M1 to CALCULATE that the basketball's velocity is unchanged in magnitude and reversed in direction, relative to earth. Then you apply the same equations again (except for M1/M2 instead of M0/M1) and apply M1 >> M2, and come to the same conclusion, relative to the basketball, and combining these to get V(M2) = - 3V(M1).
If the mass ratios were not infinite, the math would get very messy, as 2 nasty formulas would need to be cascaded!
@@RM-zz2er many comments require no response from me. If you listen to the problem and also to my solutions you will probably agree that there is no need for me to respond.
Electric charges and fields chapter there in my syllabus can you tell me here through which lecture should i go on your channel for electric charges and fields
watch my 8.02 lectures they are on this TH-cam channel
Stay blessed dear sir......
Its actually a bit like #51 is it not ? same mechanics ..
Good morning sir, I have a question, conservation of momentum is valid when the sum of all forces in the system is zero, but here it is always equal to -(m1+m2)g so what approximation are you using here to say that the net force is negligible?
If you compare the split moment before coll with the split moment after the coll there is no time for gravity to change the momentum and no time to change the KE. That's the meaning of conservation!!! watch my 8.01 lectures.
@@lecturesbywalterlewin.they9259 Thanks sir!!!
NO ONE CAN BEAT A PROFESOR WHO TEACHES ONE SUBJECT AT COLLEGE AS WELL AS EVEN AT SCHOOL LEVEL IF HE HAS MORE THAN 15 YEAR EXPERIENCE OF TEACHING !!!!!!!!!! WHY NOT YOU COME INDIA !!!!!!!! AS A GUEST LECTURE FOR STUDENTS !!!!!!!!
I was 4 times in India - My last 6 week visit was in 2014. I gave 8 lectures at 8 different Universities. March 5, 2014, a Very Special Day at IIT Kanpur in India.
voxiitk.com/stalwarts-of-physics-at-techkriti/
Good!
I used the fast solution. :-)
Hey everyone hope you’re having a good day :)
if Zeno’s observation is correct, then why is it not true in real life?
en.wikipedia.org/wiki/Zeno%27s_paradoxes
Hello Walter Sir What Physics I need to study For Machine Learning???
I don't know
@@lecturesbywalterlewin.they9259 But Sir Do you Know Physics Required For Me To Become an Quantum Physicist.It would be Nice of you SIr If you Could Help Me
@@alipoetry1626 *first get a bachelor's degree in Physics then a PhD in Quantum Physics at the best University that will accept you and take it from there*
@@lecturesbywalterlewin.they9259 Thank You Very Much Have A Nice Day
sir please make a video on how to understand physics
Ur a fool
Watch Sir's lectures
🤩
Hello everyone, I'm from Viet Nam, I love Physic
Hello Mr lewin ı took physics 801 exam today . İt was bad
where did you take it?
@@lecturesbywalterlewin.they9259 İstanbul technical university in Turkey.
I got that correct sir😁
Then u r a legend
Legends are people like Walter Lewin, Stephen Hawking not me bro